A BT transceiver/dongle may or may not be needed depending on if the computer has one built into the chipset or not.
The fact that Windows has built-in support for Bluetooth is irrelevant when coming to assess the need (or absence of a need) for a dongle.
As the OP has already said he's using Bluetooth, and he would prefer not to take up a USB port with an additional dongle, it's safe to assume he is using a Bluetooth mouse and the Bluetooth is built in to his computer.